A polyethylene terephthalate (hereinafter may be abbreviated as “PET”) is mass-produced and used as a material in a wide variety of applications such as fibers, films (including sheets), containers and electric components, due to its excellent physical properties and chemical properties. Particularly, demand for PET bottles has been rapidly increasing.
Properties required for PET differ according to applications. For example, a PET for bottles is required to be a polymer having a higher polymerization degree than those for fibers for clothing and for films (including sheets) and to have low contents of cyclic trimer and aldehyde, from the viewpoints of bottle properties (such as transparency, toughness and flavor), moldability and other properties.
The PET for bottles is generally produced by subjecting terephthalic acid and ethylene glycol to an esterification reaction and then to melt-polymerization so as to produce a prepolymer and subjecting the prepolymer to solid-state polymerization. The solid-state polymerization is effective in achieving the above high polymerization degree and low contents of cyclic trimer and aldehyde. However, since these properties generally deteriorate during the melt molding process, the above solid-state polymerization effect is impaired during production of bottles, and the required properties of bottles may not be satisfied. For example, a solid-state polymerized polymer for bottles which has an intrinsic viscosity of about 0.82 generally has a reduced content of cyclic trimer of about 3,000 ppm, and when a bottle is produced by injection-molding this polymer at 290° C., the content of cyclic trimer is increased to about 4,500 to 5,000 ppm during the melt molding process. This content limits applications of the bottle and lowers the productivity of the bottle.
The PET for bottles is generally produced by solid-state polymerizing a melt-polymerized prepolymer in a nitrogen atmosphere at temperatures ranging from 190 to 230° C. for about 20 hours. A primary reason therefor is that even if the solid-state polymerization time is extended, an effect of reducing the cyclic trimer is small, it is very difficult to reduce the content thereof to, for example, 2,500 ppm or less, and the intrinsic viscosity of the polymer becomes so high that the productivity of bottles decreases. This indicates that it is difficult to satisfy conflicting properties, i.e., reducing the content of the cyclic trimer by only solid-state polymerizing while the intrinsic viscosity of the polymer is kept within a range that secures excellent melt moldability.
Thus, a method of suppressing by-production of cyclic trimer during the melt-molding process has been studied, and several proposals have been made. For instance, as a method of suppressing by-production of cyclic trimer during the melt-molding process, a method comprising bringing a solid-state polymerized polymer into contact with hot water of 50 to 110° C. or water vapor of 70 to 150° C. to deactivate a polymerization catalyst (Patent Publication 1, Patent Publication 2, Patent Publication 3 and the like) is proposed. According to this method, there is little difference in the content of cyclic trimer between after solid-state polymerization and after melt-molding at 290° C., and an example in which a bottle having a cyclic trimer content of 2,800 to 3,900 ppm can be produced is disclosed.
Further, Patent Publication 4 discloses that a copolyethylene terephthalate copolymerized with 0.5 to 3.0 mol % of isophthalic acid (hereinafter may be abbreviated as “IPA”) and 1.0 to 2.5 mol % of diethylene glycol (hereinafter may be abbreviated as “DEG”) and having an intrinsic viscosity of 0.6 to 1.5 dl/g, a carboxyl end group concentration of not higher than 18 eq/ton and a cyclic trimer content of not higher than 0.4 wt % is a polymer which by-produces a small amount of a cyclic trimer at the time of melt-molding. As a specific example thereof (example with the smallest amount of a cyclic trimer), it is described that when a solid-state polymerized copolyethylene terephthalate copolymerized with 2.0 mol % of IPA and 2.0 mol % of DEG and having an intrinsic viscosity of 0.83 dl/g, a carboxyl end group concentration of 10.2 eq/ton and a cyclic trimer content of 0.23 wt % (2,300 ppm) was injection-molded at a cylinder temperature of 265° C., a molded article having a cyclic trimer content of 0.25 wt % (2,500 ppm) was obtained. However, this publication mentions nothing about the content of cyclic trimer when the polymer temperature is 290° C.
(Patent Publication 1) JP-A 3-47830 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
(Patent Publication 2) JP-A 8-231689
(Patent Publication 3) JP-A 6-234834
(Patent Publication 4) Japanese Patent No. 3072939